Radio frequency shielding is important for many electronic products requiring freedom from covert observation and/or to prevent interference in radio frequency apparatus caused by signals irradiating the apparatus. Examples of such equipment include secure telephony apparatus (e.g., telephones, facsimile machines, data logging and reporting devices and the like) for encrypted communication, high-security computers and electronic apparatus for metrology (e.g., electronic equipment for performing sensitive measurements, medical electronic equipment, electronic equipment for regulating explosive and/or toxic materials).
Full duplex radio-telephones and some types of compact radars, such as those employed for collision avoidance radars on vehicles and those employed for fuzing of shells, bombs and/or missiles, employ transmitting and receiving antennae in close proximity to one another. In these equipments, it is extremely desirable to shield the receiving antenna from the signal emitted from the transmitting antenna.
Metal shields are inconvenient for many applications because they do not provide high DC resistance and also because metal components are not as readily cast or shaped as many plastics are.
One method for realizing radio frequency shields of a type compatible with modern electronic equipment manufacturing practices is to modify the radio frequency properties of a plastic material suitable for molding, for example, to form an enclosure for a piece of electronic gear. Such modification may be achieved by "loading" a plastic material with finely divided conductive particles, for example, finely divided or powdered metals. In many applications, it is necessary to insulate the particles from one another to provide the desired radio frequency absorption profile and/or the desired DC conductivity characteristics. Carbonyl iron powders provide some of these characteristics, however, the resistance between grains in carbonyl iron powders may vary from lot to lot.
Some techniques for insulating conductive particles such as carbonyl iron utilize plasma processes requiring substantial capital equipment investment and which are poorly suited to low volume production of the insulated particles. In these processes, carbonyl iron is mixed with a vapor of a polymeric material which is then polymerized by application of radio frequency energy.
Another process which has been employed involves treating the carbonyl iron with phosphoric acid to oxidize the surfaces of the particles, thereby insulating the particle surfaces. This process can be unreliable and may provide decreased insulation and/or decreased resistance in some cases.
What is needed is a radio frequency absorptive plastic material which is easily and reliably produced, is readily adaptable to specific requirements, and is effective in absorption of radio frequency energy.